Should I put in my Discovery? It's a 3.9 with 81,000 on it, so I know I'm going to need some of the high-mileage stuff.

Shouldn`t need the thick stuff yet by a long way!
Stick a good-quality 10w/40 or 15w/40 in it, and it`ll be fine. 81000? Not even run in yet! Thick oil tend to take ages to fill the tappets up if any of them are a bit leaky, and there`s a lot more crank drag until the engine's warm.